铁与铜的线性膨胀系数和导热系数相差并不大，所以焊接时在接头中不会产生很大的应力。
参考答案：错
The number of people invited ____ fifty, but a number of them ____ absent for different reasons.
A．were , was
B．was , was
C．was , were
D．were , were
装有容许信号的通过信号机，显示停车信号时，准许铁路局规定停车后起动困难的货物列车，在该信号机前不停车，按运行速度不超过（）通过该信号机。
A、15km/h
B、20km/h
C、25km/h
D、30km/h
